Which of the following reacts the slowest with sodium cyanide, NaCN?

Which of the following reacts the slowest with sodium cyanide, NaCN?

A. A

B. B

C. C

D. D

Solution: NaCN consists of Na+ and CN-. CN- is a good nucleophile. The answer choices are all primary substrates. Therefore, this is an SN2 reaction. In an SN2 reaction, nucleophile attacks from the back. The more groups are in close proximity to the carbon with the leaving group, the harder it will be for the nucleophile to get to that carbon and the slower the reaction would be. In choice B, two methyl groups are closest to the carbon with the leaving group (OTs) and would hinder the nucleophile the most from being able to attack the carbon with the leaving group. Choice B is the answer.